Giardia lamblia is a phylogenetically primitive protozoan and a major cause of waterborne enteric disease throughout tropical and temperate zones. The mechanism by which G. lamblia causes diarrhea in humans is currently unknown. Despite their likely importance, relatively little is known of the molecules on the trophozoite surface or of their interactions with intestinal epithelial cells. The surface antigens which have been characterized to date are cysteine-rich proteins which vary among isolates and even between subclones of a single isolate. Although their antigenic variation is interesting and could contribute to the variability of the disease process, it is important to identify other, potentially invariant surface proteins. By biosynthetic labeling with [3H]-fatty acids, I have found that G. lamblia synthesizes a glycosylphosphatidylinositol (GPI) anchored surface protein, GP49, which differs structurally and biologically from the variable surface antigens described by others. Moreover, this surface antigen is present in different isolates and in variable subclones of a single isolate of G. lamblia and can therefore be considered as a potential candidate for chemotherapeutic as well as immunologic intervention. Accordingly, I propose to study GP49 with the following Specific Aims: (l) to confirm our preliminary evidence that it is an invariant antigen; (2) to analyze its unusual GPI anchor structure, (3) to elucidate the effect of amino sugar and inositol analogs on GPI anchor synthesis, and to analyze the intermediate glycolipids that accumulate with specific inhibitors (4) to isolate and sequence its gene in order to evaluate its structure and possible function, and (5) to study the interactions of GP49 with cultured human intestinal epithelial cell monolayers since we have found that soluble or intact GP49 elevates cytoplasmic Ca2+ concentration suggesting a possible role in secretory diarrhea. Our preliminary data suggest that these studies could shed light on the molecular pathogenesis of giardiasis and suggest immunologic approaches to this important disease.